In one prior art about the adjustable spanner, U.S. Pat. No. 2,948,175, the adjustable spanner has a transversal slot for receiving a sliding rod. A head of the adjustable spanner is installed with a wedge groove communicated to the slot for receiving a spring and a wedge block. The wedge block resists against a release rod in a receiving groove of the movable jaw. Thereby when the user press the release rod, the wedge block is pushed to move toward the fixing jaw so as to press the spring. Thus, the wedge block will separate from the sliding rod at the lower end of the movable jaw. The opening between the movable jaw and the fixing jaw is adjustable.
Furthermore, in another prior art, U.S. Pat. No. 6,848,342, a toothless adjustable spanner is disclosed. In the adjustable spanner, a spring resisting against the movable jaw and adjustable spanner is installed in the sliding groove. A receiving groove communicated to the sliding rod is installed in the head. The receiving groove is installed with a sparing and a rolling ball and a release rod a part of which exposes out. A receiving groove has an inclined surface at a direction facing the fixing jaw. Thus, when the release rod is released, the rolling ball will move into the inclined surface so that a buckling unit releases from the limitation of the movable jaw. Therefore, the size of the opening between the movable jaw and the fixing jaw is adjustable.
However the above two prior arts has the following disadvantages.
The use of the adjustable spanner is inconvenient. In the two prior arts, release rods are installed. By pressing the release rod, the wedge block or rolling ball will release and the spring fixed in the sliding groove will push the movable jaw away. However in application, a greater force is necessary to press the distal end of the movable jaw (the pressing force is greater than the elastic coefficient) so that it is installed in use. If a spring with lower elasticity is used, the spring is easily bent. As a result the lifetime of the spanner is reduced.
Furthermore, the wedge block has two sliding grooves which are engaged to two ribs of the spanner. If a greater force is applied. The two will be engaged to one another and thus it is difficult to release them.
If is difficult to communicate the wedge slot to the sliding groove. The wedge block and the ribs are difficult to manufacture. Furthermore, if a cover is used, it is difficult in machine-work and thus the cost is high.